In localities where highways or freeways adjoin residential districts or business areas, schools or hospitals, the road noise is often objectionable to the point of being intolerable to the individuals living and working in those areas. A busy freeway can generate approximately 80 dB of noise at a distance of 100 feet from the roadway if the traffic includes diesel trucks, with variations from 74 dB to 86 dB. Studies show that the average diesel truck at a distance of 25 feet, emits approximately 92 dB of noise, with some ranging as high as 105 dB. This noise varies in frequency from 60 Hz to 8 KHz, the screech of the brakes. The noise level and its frequent components are a function of the type of pavement, the type and speed of traffic, the presence of a grade, and whether the roadway is sunken, flat or elevated.
For those individuals in the vicinity of a highway, a road noise level of 65-70 dB is generally acceptable. If the noise level falls in the 70-80 dB range, complaints will be heard. At 90 dB, the road noise threatens to cause hearing damage. Since a highway is a line source of noise, the natural attenuation due to transmission loss through the air is -3 dB as the distance from the roadway doubles. Thus a truck which creates 92 dB of noise at a distance of 25 feet will cause an intolerable level of perceived noise of 80 dB at a distance of 400 feet. Obviously a great distance must separate an observer from a highway if the noise perceived by the observer is to be made tolerable through natural transmission loss.
Clearly there is a need for a noise attenuating structure to separate highways and freeways from adjoining dwellings. One such structure which is commonly used is an earth berm planted with grass, flowers, trees and shrubs. Although such an earth berm is aesthetically pleasing, it typically requires 32 feet of additional right-of-way for the road, plus 5 cubic yards of fill material per linear foot. In addition the plantings must be watered and maintained. An earth berm cannot be used along an elevated roadway, nor in an urban area where a narrow right-of-way is an economic necessity. Other structures similar in nature but fabricated of wood, stone, brick, concrete or the like have similar drawbacks and limitations.